Corrosion inhibitors for petroleum industry

Corrosion inhibitors for the petroleum industry are substances used in the oil industry to protect equipment and pipelines against corrosion. Corrosion is a daily problem in the oil industry due to the presence in crude oil of water contaminated with salts, gases and other corrosive contaminants in the production process. Corrosion inhibitors can be classified according to their chemical composition as either organic inhibitors or inorganic inhibitors. They can also be classified by the way they act as anodic or cathodic inhibitors. Cathodic inhibitors act as catalysts to slow down corrosion, while anodic inhibitors protect metal surfaces by acting as physical barriers.

Corrosion inhibitors are used in different processes in the petroleum industry, such as well drilling, pipeline transportation in and out of the refinery, fuel production, in water treatment systems and in oil and gas storage.

Corrosion inhibitors come in a variety of chemical families, some of which are used in the oil industry. Acid inhibitors, sulfide inhibitors, oxygen inhibitors, and corrosion-promoting microorganism inhibitors are a few types of corrosion inhibitors used in the oil industry. They can also be classified by their chemical composition into organic and inorganic corrosion inhibitors.

Some chemical families of corrosion inhibitors used in the oil industry

There are different chemical families of corrosion inhibitors used in the oil industry, among them are the following:

Fatty imidazolines: These are imidazole-based compounds, with long unsaturated chains, derived mainly from oleic acid. They are mainly used to prevent acid corrosion of carbon steel. They form a filmic layer inside pipes and equipment, reducing the interactions between the fluid and the metal (Figure 1).

Figure 1. Example of fatty imidazolines.

Fatty amines: These corrosion inhibitors are nitrogenous organic compounds containing an amino group and a long-chain alkyl group (more than 12 carbon atoms). They act as cathodic inhibitors and form a protective layer on the metal surface. They are also effective against corrosion caused by carbon dioxide. (CO₂) and hydrogen sulfide (H₂S). Also, ethoxylated amines are used for the same purpose (Figure 2).

Figure 2. Example of fatty amines and ethoxylated amines.

Organic acids: Organic acids such as acetic acid, formic acid and citric acid are used as corrosion inhibitors. These acids react with metal ions to form insoluble compounds that protect the metal surface. These inhibitors are often used in combination with other corrosion inhibitors and techniques, such as cathodic protection and coatings, to provide comprehensive corrosion protection. They are effective at controlling corrosion caused by CO₂ and H₂S, which are commonly found in petroleum reservoirs and can be highly corrosive. (Figure 3).

Figure 3. Example of organic acids used as corrosion inhibitors

Pyridines: Some studies have shown that certain pyridines can inhibit corrosion caused by the presence of acid gases, such as CO₂ and H₂S, which are common in the oil industry. Pyridine and its derivatives have been shown to be effective inhibitors for a wide range of metals, such as carbon steel, stainless steel, and copper alloys. They act by adsorbing to the metal surface and forming a protective film, which can be physical or chemical in nature. Pyridine and its derivatives are also effective in inhibiting localized corrosion, such as pitting and crevice corrosion (Figure 4).

Figure 4. Example of pyridines employed as corrosion inhibitors.

Azoles: Azoles, such as triazole and benzotriazole, oxazole and benzoxales, and thioazoles and benzothioazoles are organic compounds used as corrosion inhibitors in the petroleum industry. They act as anodic inhibitors and form a protective layer on the metal surface (Figure 5).

Figure 5. Example of benzoazoles and triazoles.

Polymers: Polymers are large molecules with high molecular weight used in the petroleum industry as corrosion inhibitors. Some polymers can adsorb onto the metal surface and form a protective coating. They can also be used as dispersants to prevent the formation of corrosive deposits. Some examples are:

Aminated polymers: These polymers are used for corrosion protection of metal surfaces in the oil industry. They are highly effective in preventing salt water and H₂S corrosion.

Acrylic polymers: These polymers are used as corrosion inhibitors in the industry due to their good compatibility with oil and drilling fluids. They are effective against corrosion caused by the presence of hydrochloric acid (HCl) in drilling fluids.

Maleate polymers: These polymers are used as corrosion inhibitors in the industry due to their good adsorption capacity on metal surfaces and their high solubility in oil and drilling fluids. They are effective against corrosion caused by the presence of H₂S in drilling fluids. (Figure 6).

Figure 6. Example of polymers used as corrosion inhibitors.

Other organic products used as corrosion inhibitors in oil industry are nitriles, amides, oximes, ureas and thioureas, phosphonate salts. Inorganic inhibitors such as lanthanides, molybdates, silicates, boric and phosphoric acid, and the combination of nitrates and nitrites are also widely employed, as well as more environmentally friendly inhibitors such as some biomass wastes, amino acids, and ionic liquids.

These inhibitors work by forming a protective layer on the surface of the metal, which prevents acids and other corrosive chemicals from coming into direct contact with the metal. It is critical to choose the right corrosion inhibitor based on the type of corrosion present, environmental conditions, temperature, pressure and type of metal to be protected, and to ensure that the right amount is applied for maximum protection. Periodic corrosion rate monitoring should also be performed to confirm that our inhibitor is working properly.